Problem is it isn’t state vs state, it’s urban vs rural.
For example, NY is really quite “red”, except for concentrated pockets of “blue” which outvote them.
My usual suggestion is to start at the state-line swamps and follow the Croton River down to its outfall into the Hudson (this puts Chappaqua, btw, conveniently on the Connecticut/downstate side of the line), and then down the center of the Hudson River channel and canyon out to the toe of the continental rise. (So to do will be necessary to divide mineral rights in the Outer Continental Shelf.)
We get Staten Island, Bedloe's, Ellis Island, and all of Jersey and upstate New York. They get Riker's, the other boroughs, Westchester, and New England. The international boundary would be the New York state line -- up to Lake Champlain, where it'd probably be an idea to split the lake islands carefully.
Then we drop the GW and Tappan Zee bridges into the drink, flood the tunnels, and put a big-ass turnstile on the Verrazano Narrows Bridge, and call out "Geh veck vom mein Hause!" and break the china. All done.
Add King County in Washington to the list. We in the Conservative east side have no voice.